It has long been known that the introduction of surface "impurities" into metals can have beneficial effects. Surface hardening, as it is known in many references, can be utilized to provide a hard surface which is resistant to wear, corrosion and abrasion while retaining the ductile interior composition of the metal part and retain resistance to fracture and the like. It is desirable to utilize surface hardening techniques in a variety of applications, particularly with respect to parts which are exposed to abrasion and/or caustic and high temperature environments.
The most common example of surface treatment of metals, which has been known for many decades, is in providing a surface hardening to steel. This method, which is typically known as carburizing, is utilized to embed atomic carbon into the metallic matrix of the steel component near the surface. Typically, the surface penetration is every limited, with the usual penetration being in the range of 0.1 cm or less. The absorption of carbon into the steel surface is well known and has been described in a variety of metallurgical references, including Elementary, Metallurgy and Metallography, by Arthur M. Shrager, Dover Publications Inc., at pages 175 et seq.; in Principles Of The Surface Treatment Of Steels, by Charlie R. Brooks, Technomic Publishing Company Inc., at pages 67 et seq., in Carburizing and Carbonitriding, by American Society for Metals 1977, and in Carburizing Process and Performance, edited by George Krauss, ASM International 1989.
Carburizing of steel is typically conducted in either a gaseous atmosphere (gas carburizing), a carbon powder bed (pack carburizing), or a molten salt bath containing carbon (liquid carburizing). The primary carbon transport species for these processes is carbon monoxide.
Gas carburizing involves exposing steel to a gas mixture containing carbon monoxide (CO), hydrogen gas (typically methane (CH.sub.4) hydrogen (H.sub.2), and Nitrogen (N.sub.2). The carbon monoxide, hydrogen, and methane react with the surface of the steel allowing the dissolution of carbon. The reactions which are directly responsible for carbon deposition are: EQU Fe+2CO=Fe(C)+CO.sub.2 EQU Fe+CH.sub.4 =Fe(C)+2H.sub.2 EQU Fe+CO+H.sub.2 =FE(C)+H.sub.2 O.
In addition to providing carbon directly, methane also reduces the partial pressures of CO.sub.2 and H.sub.2 O, both of which decarburize steel, in the reaction vessel. This occurs via the reactions: EQU CH.sub.4 +CO.sub.2 =2CO+2H.sub.2 EQU CH.sub.4 +H.sub.2 O=CO+3H.sub.2.
Nitrogen acts as an inert carrier gas. Typical gas carburizing process temperatures are in the range of 850.degree. to 950.degree. C.
Pack bed carburizing involves covering the steep with finely divided carbon powder and heating to 800.degree. to 1100.degree. C. Carbon monoxide gas formed by the decomposition of the carbon powder transports the carbon to the surface of the steel.
The liquid carburizing process uses a high temperature (900.degree. C.) molten salt bath containing carbon powder. The reaction of the molten carbonate and the carbon produces carbon monoxide which is transported to the surface of the steel.
When performing on steels, carbonizing is a modified form of gas carburizing. The steel is exposed to an atmosphere containing both carbon and nitrogen at temperatures of 700.degree. to 900.degree. C.; where both the carbon and nitrogen are absorbed into the steel simultaneously. Ammonia (NH.sub.3) is introduced to the gas carburizing atmosphere to add nitrogen to the metal being processed. Liquid carbonitriding is also performed using cyanides (sodium cyanide) in a molten salt bath.
Refractory metals are typically carburized in hydrocarbon gas environments (G. Horz and K. Lindenmaier, "The Kinetics and Mechanisms of the Absorption of Carbon by Niobium and Tantalum in a Methane or Acetylene Stream," Journal of the Less Common Metals, 35 (1974), pp. 88-95). They are processed differently from steels because they tend to form oxides, rather than carbides, when exposed to carbon monoxide. Oxide formation passivates the surface, preventing further carbon absorption. This behavior is also seen in steel containing significant quantities of chronium and silicon. Since refractory metals have a high affinity for oxygen, they are usually carburized and carbonitrided in vacuum furnaces.
Pack carburizing has also been performed on refractory metals (R. L. Andelin, L. D. Kirkbride, and R. H. Perkins, "High-Temperature Environmental Testing of Liquid Plutonium Fuels," Los Alamos National Laboratory Report LA-3631, 1967).In this work, refractory metal tubes were packed with carbon granules, heated in vacuum to 1700.degree. C. and then filled with hydrogen. After five minutes, the hydrogen was pumped out and the tube cooled to room temperature in helium. Hydrogen is introduced so that it may react with the carbon and produce hydrocarbons. The hydrocarbons then react with the metal to produce a carbide.
There are also methods used to carbonitride refractory metals. The parts are placed in a pure carbon bed and heated in a nitrogen atmosphere at temperatures in the range of 1200.degree. to 1600.degree. C. It was believed that some of the carbon in contact with the metal was able to diffuse into the metal at the same time that nitrogen was absorbed from the gas phase.
Two methods of applying a carbide coating are described in U.S. Pat. No. 4,150,905, issued Apr. 24, 1979 to Kaplan et al. and U.S. Pat. No. 4,430,170, issued Feb. 7, 1984 to Stern. These references include a discussion of the problems and purposes of the coating technology and also describe some of the previous attempts at accomplishing this. The Kaplan reference describes a method of applying vapor depositions of a separate layer of material on the exterior of a ball shaped element, particularly the ball for a ball point pen. The method is shown as being particularly intended for a deposition of a layer of tungsten carbide on the exterior of a ball formed of tungsten or a variety of other materials.
The Stern patent utilizes an electro deposition technique with an alkali fluoride melt acting as the electrolyte. In such a case, a deposit of a layer of metal carbide can be applied to a desired thickness on any of a variety of appropriate materials. The Stern reference describes successful efforts with a variety of refractory metals, including results showing less success with respect to chromium.
Accordingly, much room for improvement remains in the art with respect to surface treatment for refractory metals in order to provide strong integral abrasion and corrosion resistant surfaces while avoiding contamination of the properties of the item itself. A strong need remains for metallic parts formed from refractory metals which are provided with such types of surfaces.